In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to direct fuel pulses into the engine combustion chamber. A commonly used injector is a closed-nozzle injector which includes a nozzle assembly having a spring-biased nozzle valve element positioned adjacent the nozzle orifice for resisting blow back of exhaust gas into the pumping or metering chamber of the injector while allowing fuel to be injected into the cylinder. The nozzle valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The nozzle valve is positioned in a nozzle cavity and biased by a nozzle spring to block fuel flow through the nozzle orifices. When the pressure of the fuel within the nozzle cavity exceeds the biasing force of the nozzle spring, the nozzle valve element moves outwardly to allow fuel to pass through the nozzle orifices, thus marking the beginning of injection.
Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. It is well known that the level of emissions generated by the diesel fuel combustion process can be reduced by decreasing the volume of fuel injected during the initial stage of an injection event while permitting a subsequent unrestricted injection flow rate. As a result, many proposals have been made to provide injection rate control devices in closed nozzle fuel injector systems. One method of controlling the initial rate of fuel injection is to spill a portion of the fuel to be injected during the injection event. For example, U.S. Pat. No. 5,647,536, entitled Injection Rate Shaping Nozzle Assembly for a Fuel Injector and commonly assigned to the assignee of the present application discloses a closed nozzle injector which includes a spill circuit formed in the nozzle valve element for spilling injection fuel during the initial portion of an injection event to decrease the quantity of fuel injected during this initial period thus controlling the rate of fuel injection. A subsequent unrestricted injection flow rate is achieved when the nozzle valve moves into a position blocking the spill flow causing a dramatic increase in the fuel pressure in the nozzle cavity. Other rate shaping systems decrease rate of fuel flow during the initial portion of the injection event by, for example, throttling the fuel to the nozzle orifices. Although these systems create injection rate shaping, the spilling and throttling of fuel during the initial period of injection achieves a reduced injection flow rate by reducing the injection pressure adjacent the nozzle orifices. The decrease in injection pressure may disadvantageously result in decreased atomization of the fuel spray by the nozzle orifices, thus adversely affecting fuel economy and increasing emissions.
U.S. Pat. No. 5,199,398 to Nylund discloses a fuel injection valve arrangement for injecting two different types of fuels into an engine which includes inner and outer poppet type nozzle valves. During each injection event, the inner nozzle valve opens a first set of orifices to provide a preinjection and the outer nozzle valve opens a second set of orifices to provide a subsequent main injection. The outer poppet valve is a cylindrical sleeve positioned around a stationary valve housing containing the inner poppet valve.
U.S. Pat. No. 4,546,739 to Nakajima et al. discloses a fuel injector with inner and outer injector nozzle valves biased to close respective sets of spray holes and operable to open at different fuel pressures. The inner nozzle valve is reciprocally mounted in a central bore formed in the outer nozzle valve. However, the nozzle valves are controlled such that both are open simultaneously at high engine speeds while only one is opened at low speeds, and therefore, these valves are not both opened during a single injection event to achieve two stage injection.
U.K. Patent Application No. 2266559 to Hlousek discloses a closed nozzle injector assembly including a hollow nozzle valve for cooperating with one valve seat formed on an injector body to provide a main injection through all the injector orifices and an inner valve nozzle reciprocally mounted in the hollow nozzle for creating a pre-injection through a few of the injector orifices. However, the inner valve element opens and closes to provide a separate pre-injection event and therefore does not function to shape the primary injection. Moreover, the valve seat allowing the inner valve nozzle to block the pre-injection flow is formed on the hollow valve member and the inner valve nozzle is biased outwardly away from the injector orifices. This arrangement requires a third valve seat for cooperation with the inner valve element when its in a pre-injection open position to prevent flow through all of the injector orifices, resulting in an unnecessarily complex and expensive assembly. Also, this assembly is designed for use with two different sources of fuel requiring additional delivery passages in the injector.
Consequently, there is a need for a fuel injector incorporating a simple, cost effective nozzle assembly capable of effectively and reliably creating a low injection flow rate during an initial stage of an injection event to thereby control emissions.